To adjust speed and reduce gasoline consumption, every means of transportation nowadays is equipped with a gear shift mechanism. A conventional gear shift mechanism essentially comprises a gear train or essentially comprises a gear train and an oil duct. However, the gear train or the combination of the gear train and the oil duct is structurally intricate and bulky, has a narrow gear-changing range, incurs much transmission loss, and tends to jerk while shifting gear. Therefore, a stepless gear shift mechanism characterized by two grooved wheels operating in conjunction with a V-shaped belt is developed. However, the stepless gear shift mechanism has disadvantages, namely large volume of the grooved wheels and the V-shaped belt, and a narrow gear-changing range. Accordingly, the present invention aims to disclose a linear gear shift mechanism which is structurally simple and compact, has a wide linear gear-changing range, incurs little transmission loss, and never jerks while shifting gear.